James -
I've just been reviewing the RFC 1738 (URL spec, mostly in context of some formal
comments that I putting together about the 2.2 spec). From that RFC:
A file URL takes the form:
file://<host>/<path>
where <host> is the fully qualified domain name of the system on
which the <path> is accessible, and <path> is a hierarchical
directory path of the form <directory>/<directory>/.../<name>.
As a special case, <host> can be the string "localhost" or the empty
string; this is interpreted as `the machine from which the URL is
being interpreted'.
The file URL scheme is unusual in that it does not specify an
Internet protocol or access method for such files; as such, its
utility in network protocols between hosts is limited.
And from the BNF at the end of the RFC:
fileurl = "file://" [ host | "localhost" ] "/" fpath
So, according to the spec, both of
file://localhost/PATH
file:///PATH
a equal. The BNF further implies that URL are absolute: There is no way to specify a
resource otherwise.
Now, RFC 1630 (URI spec) does allow for relative URIs:
Partial (relative) form
Within a object whose URI is well defined, the URI of another object
may be given in abbreviated form, where parts of the two URIs are the
same. This allows objects within a group to refer to each other
without requiring the space for a complete reference, and it
incidentally allows the group of objects to be moved without changing
any references. It must be emphasized that when a reference is
passed in anything other than a well controlled context, the full
form must always be used.
The rules for the use of a partial name relative to the URI of the
context are:
If the scheme parts are different, the whole absolute URI must
be given. Otherwise, the scheme is omitted, and:
If the partial URI starts with a non-zero number of consecutive
slashes, then everything from the context URI up to (but not
including) the first occurrence of exactly the same number of
consecutive slashes which has no greater number of consecutive
slashes anywhere to the right of it is taken to be the same and
so prepended to the partial URL to form the full URL. Otherwise:
The last part of the path of the context URI (anything following
the rightmost slash) is removed, and the given partial URI
appended in its place, and then:
Within the result, all occurrences of "xxx/../" or "/." are
recursively removed, where xxx, ".." and "." are complete path
elements.
So, if you've got a "URI in context", you can use relative URIs. Some examples (from
the spec):
In the context of URI
magic://a/b/c//d/e/f
the partial URIs would expand as follows:
g magic://a/b/c//d/e/g
/g magic://a/g
//g magic://g
../g magic://a/b/c//d/g
g:h g:h
and in the context of the URI
magic://a/b/c//d/e/
the results would be exactly the same.
This is nice, but it really only gives you a way to translate relative URIs into
absolute URLs, based on the URI context.
So basically, what I'm saying is that there is currently no way to specify a "relative
file url", mostly because for the URL to be relative, it has to have some sort of
context. Basically, I think it comes down to is using ".." in an absolute URL and
generating that URL:
String url;
String file = "../foo.jar";
String base = getCodeBase();
if ( base.endsWith( "/" ) ) {
url = base + file;
} else {
url = base + "/" + file; // this might always be valid;
}
This is why I always use getResource(). Perhaps there should be a File() constructor
that takes a URL, or developers should consistantly use getResource() and
URL.getFile().
- Paul Philion
James Todd wrote:
>
> [i'm going to cross-post my reply to the jsp and servlet
> interest lists ... as i've been burned on this very item
> in the past and am curious as to folks opinions].
>
> hi joe -
>
> !great information!
>
> for a good part of the day i've been trying to resolve
> a "portable file url specified docBase" issue. for what
> it is worth and this may be a bit dated but i've used
> the following format since jdk 1.0.x days with measurable
> success:
>
> file://localhost/PATH
>
> where PATH can look like "foo/bar" for unix
> and "C|/foo/bar" (or something like this)
> on win
>
> i've run into modo various jdk version/platform issues
> when using file urls which are specified with anything
> less then the above. that said, i'm really not sure how
> to specify relative file urls with the above scheme in
> a portable manner as the "third" slash designates the
> the root file system ... or so i believe.
>
> eg: file:../foo.jar
>
> i'd like to hear thoughts on this item as it is really
> a bit of a pain in the a$$ when it comes to specifying
> a reliable (ie jdk 1.1.x and jdk 1.2.x on win/solaris/*
> for relative and absolute paths) file based docBase.
>
> any ideas? i am optimistic (and stubborn) so i believe
> there is a solution here.
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